Accelerated in vitro release testing methods for extended release parenteral dosage forms

Abstract

Objectives

This review highlights current methods and strategies for accelerated in vitro drug release testing of extended release parenteral dosage forms such as polymeric microparticulate systems, lipid microparticulate systems, in situ depot-forming systems, and implants.

Key findings

Extended release parenteral dosage forms are typically designed to maintain the effective drug concentration over periods of weeks, months or even years. Consequently, “real-time” in vitro release tests for these dosage forms are often run over a long time period. Accelerated in vitro release methods can provide rapid evaluation and therefore are desirable for quality control purposes. To this end, different accelerated in vitro release methods using United States Pharmacopoeia (USP) apparatus have been developed. Different mechanisms of accelerating drug release from extended release parenteral dosage forms, along with the accelerated in vitro release testing methods currently employed are discussed.

Conclusions

Accelerated in vitro release testing methods with good discriminatory ability are critical for quality control of extended release parenteral products. Methods that can be used in the development of in vitro-in vivo correlation (IVIVC) are desirable, however for complex parenteral products this may not always be achievable.

Introduction

Extended release parenteral dosage forms (such as polymeric microparticulate systems, lipid microparticulate systems, in situ depot-forming systems, and implants) have attracted extensive attention during the past decades.^[1-6] Such systems can maintain effective drug concentrations over extended periods of time, minimize undesirable fluctuations in systemic drug concentration and reduce administration frequency, thus improving patience compliance.^[7] Since MR parenteral dosage forms usually contain substantial amounts of potent therapeutic agents, dose dumping or unanticipated changes of in vivo drug release characteristics may lead to severe side effects.^[8] Accordingly, it is essential to understand in vivo performance and have appropriate in vitro release testing methods that can mimic the in vivo performance of these systems.

In vitro release testing methods with good discriminatory ability are critical for quality control purposes and the development of methods that can be used in in vitro-in vivo correlation (IVIVC) are desirable to assist in formulation development and help reduce the regulatory burden of bioequivalence testing. It has been more than 100 years since the first dissolution test was introduced by Noyes and Whitney.^[9] In 1970, the United State Pharmacopeia (USP) adopted the basket-stirred-flask test (USP apparatus 1) as the first official dissolution test method for solid oral dosage forms.^[10] Since then, dissolution testing has become an essential quality control test. The dissolution test is referred to as an “in vitro drug release” test in the case of extended release parenteral dosages forms.^[11] In general, compendial apparatus and methods should be used as a first approach in drug development. However, unlike conventional solid oral dosage forms, extended release parenteral dosage forms have very wide array of physicochemical and release characteristics. Considering the diversity of such systems, it is very challenging to develop regulations and standards. Consequently, at this time, there are no standard compendial method(s) for in vitro release testing of extended release parenteral release dosage forms.

Various in vitro release testing methods (such as sample-and-separate, dialysis sac, and continuous flow methods) have been used for extended release parenteral dosage forms.^[12-16] Extended release parenteral dosage forms such as microspheres are typically designed to maintain the effective drug concentration over periods of weeks, months or even years. Therefore, “real-time” in vitro release studies for these dosage forms would require extended periods of time, which would impact the time to batch release of product and hence reduce the effective product shelf life. Over the past few years, accelerated in vitro release methods have received considerable attention in order to shorten the time required to study drug release.^{[17; 18]} Parameters that can be utilized to achieve accelerated release include: temperature, solvent, ionic strength, pH, enzymes, surfactants as well as agitation rate.^[19] However, such accelerated conditions may not only accelerate the rate of drug release but also change the mechanism of drug release.^{[20; 21]} Therefore, it is very important to understand the drug release mechanism as well as how accelerated parameters may affect it.

Ideally, drug release from “real-time” and accelerated tests should follow the same release mechanism with a 1:1 correlation between the release profiles.^[22] However, it is possible that the drug mechanism(s) may change since accelerated release tests are typically performed under extreme conditions (e.g. high temperatures as well as acidic or basic pH conditions). Nevertheless, “real-time” and accelerated release profiles should show a minimum of a rank order relationship between different formulations.^[19] Accelerated release testing should be capable to serve as a discriminatory tool as long as all formulations experience similar changes and continue to exhibit performance characteristics that can be differentiated from each other. It is recommended that the specifications for accelerated release should include a determination of at least 80 % of the cumulative amount released for comparison with “real-time” studies.^[23]

This review will summarize in vitro drug release mechanism(s) of commonly investigated extended release parenteral dosage forms, followed by a discussion of current accelerated in vitro release testing methods for these systems.

Mechanism(s) of in vitro drug release from extended release parenteral dosage forms

The drug release mechanisms from extended release parenteral dosage forms may vary depending on the characteristics of the dosage forms as well as the conditions and the methods used for in vitro release testing. Therefore, understanding the in vitro drug release mechanism can facilitate the development of a suitable accelerated in vitro release testing method.

Polymeric microparticulate systems

Polymeric microspheres are one of the most commonly investigated extended release parenteral dosage forms. These systems can be administrated via subcutaneous or intramuscular injection as well as by direct injection into the target site or tissue. Since microspheres cannot be retrieved following parenteral administration, the polymers used in formulation design should be biodegradable as well as biocompatible.^[24] A variety of biodegradable polymers, including lactide/glycolide polymers (such as polylactic acid (PLA) and poly (lactic-co-glycolic acid) (PLGA)), poly-ε-caprolactone, polyanhydrides, polyortho esters, and albumin, have been utilized to prepare microparticles.^{[25; 26; 24; 27]}

Based on the physicochemical characteristics of the polymer, drug release from polymeric microparticles may involve diffusion, polymer erosion or a combination thereof.^{[28; 29]} It is known that diffusion governs the initial burst release phase, while polymer erosion is dominant in the later primary release phase.^{[28; 30; 31]} Fick’s second law of diffusion can be used to elucidate the diffusion-controlled drug release phase.^[32] In the case of the erosion-controlled release phase, drug release is mainly controlled by polymer erosion resulting from hydrolysis of the polymer chains. As shown in Figure 1, there are two different polymer erosion scenarios: surface (heterogeneous) and bulk (homogeneous) erosions.^[33] In surface erosion, polymer (e.g. polyanhydrides or polyortho esters) erosion takes place at the external matrix boundary, which may result in a decreasing microsphere diameter.^{[29; 34]} In bulk erosion, polymer (e.g. poly lactide/glycolide polymers) degradation takes place throughout the entire microsphere structure.^[33] Several parameters including polymer properties (e.g. molecular weight, copolymer composition, and crystallinity),^{[35; 36]} drug properties,^[37] particle size,^{[38; 39]} as well as release conditions (such as release media and agitation),^[40] can affect drug release from polymeric microspheres.

Figure 1.

Commonly used in vitro release testing methods for polymeric microparticles include: sample-and-separate methods, continuous flow cell methods, and dialysis methods.^[14]

Lipid microparticulate systems

Lipid microparticulate systems (such as oil suspensions, multivesicular liposomes, and lipid microparticles) can be injected subcutaneously, intramuscularly or intra-articularly to achieve sustained drug delivery.^{[41; 42]} Lipid nanoparticles (small unilamellar liposomes as well as solid lipid nanoparticles) can also be injected intravenously. In general, the duration of release from lipid microparticulate systems is no longer than one week. This can be compared to polymeric microparticulate systems which can be programmed to release anywhere from a few days to years.^[24] Although lipid microparticulate systems have been in clinical use for several decades in the field of schizophrenia and hormone replacement therapy,^{[43; 44; 42]} there is some controversy over the drug release mechanism(s) from such systems.

DepoFoam™ is a lipid-based drug delivery system consisting of microscopic, spherical particles (10-20 μm) with hundreds of nonconcentric aqueous chambers. Drug release from DepoFoam™ is controlled by drug diffusion through the phospholipid bilayers.^[45] In the case of oil suspensions, two drug release mechanisms have been proposed: i) the suspended drug dissolves in the oil phase prior to release into the aqueous phase via partitioning;^{[46; 47]} and ii) the solid drug particles in the oil phase are transported via sedimentation to the oil-water interface or directly into the aqueous media where the drug undergoes dissolution/release.^[48]

In vitro release testing methods used for the lipid-based microparticulate systems can be divided into three categories: i) lipophilic solution floating on the top of the release medium;^[47] ii) dialysis techniques;^{[49; 41]} and iii) continuous flow cell methods.^[42] Different experimental setups can influence oil-water interfacial area, hydrodynamics (stirring conditions) and sink conditions, thus affecting drug release from such systems.^[42]

In situ depot-forming systems

In situ depot-forming systems consist of a biodegradable carrier dissolved in a suitable solvent in which the drug is either dispersed or dissolved. Following parenteral administration (e.g. subcutaneous, intratumoral or intramuscular injection), a depot is formed at the site of injection to achieve sustained drug release over several days or months.^[50] The mechanism of depot formation can be classified into:^{[51; 52]} 1) thermoplastic pastes; 2) in situ cross-linked polymer systems; 3) in situ polymer precipitation; 4) thermally induced gelling systems; 5) organogels; and 6) hydrophobic fatty acid-based injectable pastes.

Drug release from in situ depot-forming systems is typically controlled by diffusion as well as a combined mechanism of diffusion/polymer erosion.^{[50; 53; 54]} The hydrophobicity and concentration of the biodegradable carrier, the polar nature/water miscibility of the organic solvent as well as the aqueous solubility and loading of the drug affect the drug release rate.^[24]

For in situ forming systems, most in vitro release tests have been performed using variants of the sample-and-separate methodology.^[55] In addition, the dialysis method has also been used to evaluate in situ depot-forming formulations.^[56] When performing an in vitro release test, the pre-gelled formulation is usually held in a special retainer to achieve a defined geometry or formulation-buffer interfacial area. Alternatively, the already formed formulation can be placed into the release medium using a syringe.^{[57; 58]}

Implantable systems

Typically, implantable systems are inserted into specific body sites by minor surgical procedures or simple injection. Unlike the polymeric microparticulate systems, both biodegradable and non-biodegradable materials can be used to prepare implants. In the case of non-biodegradable implants, a second surgical procedure is required to remove the implants.^[24]

The drug release mechanism from implants is complex and depends on various factors, including the type and amount of material used (e.g. biodegradable polymers, lipids or nonbiodegradable polymers), the properties of incorporated drug as well as the preparation techniques.^[59] Similar to other biodegradable polymer-based dosage forms, drug release from polymeric implants is mainly controlled by diffusion or a combination of diffusion and polymer erosion.^[5] Lipid (e.g. triglycerides, monoglycerides, and fatty acids) implants have been used as alternative carriers for sustained protein delivery to avoid the creation of acidic micro-climates associated with hydrolysis degradation of biodegradable polymers (e.g. PLGA).^[60-62] It has been reported that drug release from lipid implants is typically controlled by diffusion.^[61] In addition, the swelling process as well as the addition of release modifiers (e.g. PEG) may also play important roles on drug release from lipid implants.^[63-65] Nonbiodegradable polymers or biodegradable elastomers were utilized along with osmotic pressure to achieve precise zero-order drug delivery kinetics.^{[66; 67]} These osmotically controlled implants are different from other implantable systems that rely on diffusion or polymer erosion for sustained drug delivery.

To study the in vitro drug release from the implantable systems, the implants are usually placed into glass vials with or without agitation.^{[66; 61; 63]} In addition, a flow-through apparatus has also been used to determine drug release from such systems.^[68-70]

Parameters accelerating in vitro drug release

Based on a knowledge of the drug release mechanisms from the extended release parenteral dosage forms, it is known that several parameters (such as temperature, pH, surfactant, agitation rate, and presence of enzymes) can affect the drug release profile (e.g. hasten the rate of polymer hydration and degradation or enhance drug diffusion), thus accelerating drug release.^[71-73]

Temperature

Elevated temperature has been widely used to accelerate drug release from the extended release parenteral dosage forms.^{[18; 22; 15; 74; 70]} High temperature can increase molecular mobility. At temperatures above the polymer glass transition temperature (Tg), the increased polymer mobility results in significant acceleration of drug release via diffusion.^{[18; 22]} Furthermore, high temperature can enhance hydration and degradation of polymers, thus accelerating erosion-controlled drug release.^[22]

Ideally, accelerated tests should be predictive of “real-time” release tests.^{[19; 21]} The Arrhenius equation (Equation 1) has been used to determine whether drug release rates at elevated temperatures can be used to predict “real-time” release.^{[75; 22; 70]}

$$k=A\times e^{-Ea∕RT}$$ (1)

where k is zero-order release rate, A is a constant, E_a is the energy of activation, R is the gas constant (cal/deg mol) and T is the absolute temperature.

Taking the natural logarithm of the Arrhenius equation yields:

$$\ln \left(k\right)=-\frac{Ea}{R}\times \frac{1}{T}+\ln \left(A\right)$$ (2)

The rate constant (k) at different elevated temperatures can be calculated based on the release data. A plot of ln(k) versus 1/T gives a straight line, whose the slope is −E_a/2.303R, where E_a is the energy of activation.

As shown in Figure 2, dexamethasone release rates from PLGA microspheres at elevated temperatures were successfully used to predict “real-time” release applying the Arrhenius equation.^[22] The predicted “real-time” rate constant at 37°C (solid circle) was in agreement with the experimental value at 37°C (open circle).

Figure 2.

In the case of polymeric microparticles, the initial burst release phase that is governed by diffusion changed significantly at elevated temperature. However, elevated temperature accelerated testing failed to accurately predict the “real-time” burst release phase due to a combination of two competing factors.^[76] Elevated temperature can increase polymer mobility, thus resulting in increased drug release via diffusion. Meanwhile the increased polymer mobility can cause microsphere surface morphology changes (e.g. pore closure), which in turn may decrease drug release.^[22] Therefore, it is recommended that elevated accelerated release tests should be augmented by an initial “real-time” study that allows adequate assessment of any burst release phase.^[23] In addition, it should be noted that high temperature may result in accelerated degradation of media components as well as the drug.^[11]

pH

pH is another important parameter that can affect the hydrolysis kinetics of biodegradable polyesters, resulting in accelerated drug release from these systems (e.g. polymeric microparticulate systems or polymeric implants).^[34] It is well known that PLGA is degraded by non-enzymatic hydrolysis of the ester backbone under physiological conditions (as shown in Figure 3). Accordingly, both acidic and basic conditions can accelerate degradation of such polymers. However, the polymer erosion mechanism appeared different under these two pH conditions.^{[77; 78]} Under acidic conditions, PLGA erosion followed a bulk erosion profile that was similar to the degradation characteristics obtained at pH 7.4.^[33] Whereas under basic conditions (pH>13), degradation appeared to occur by surface erosion.^[33] It has also been demonstrated that morphology changes in the microspheres at acidic pH is considerably different from that at pH 7.4. This may be due to a more homogeneous degradation pattern at pH 2.4.^[34]

Figure 3.

Although extreme pH conditions can hasten drug release, the acceleration of drug release is not as significant as that achieved at high temperatures. Additionally, extreme pH conditions may not be suitable for the drugs that are sensitive to these extreme pH conditions.

Release media

Accelerated drug release from the extended release parenteral dosage forms can also be achieved by adding surfactants or organic solvents into the release media. In the case of lipid implants, the presence of surfactants (e.g. Tween 20) in the release media can facilitate wetting and buffer penetration, and/or increase drug solubility in the media (via micelle solubilization), resulting in faster drug release.^{[61; 79]} Moreover, some surfactants (e.g. 0.1% Tween 81) may interact with lipid matrix and induce the formation of cracks in the lipid matrix, thus accelerating drug release.^[79]

Organic solvents (e.g. ethanol, acetonitrile) have been successfully used to achieve accelerated drug release.^{[80; 73]} The addition of acetonitrile to the release media can increase the porosity of PLGA-based stent matrices and therefore result in accelerated drug release. This method has been used to discriminate different variables in the manufacturing process and a good correlation with “real-time” release was shown.^[73]

Other parameters

Changing other parameters (such as the agitation conditions, and the interfacial area) that have an influence on the in vitro drug release characteristics can also accelerate drug release from extended release parenteral dosage forms.^{[81; 82]} For example, the drug oil-water distribution coefficient is a key parameter influencing drug release from oil depot formulations. Accordingly, a rotating dialysis cell model that generates a high oil-buffer interfacial area can accelerate drug release from such formulations.^[82]

Current in vitro accelerated release models

In contrast to oral and transdermal extended release dosage forms, no standard pharmacopeial or other regulatory method exists for in vitro drug release testing of extended release parenteral dosage forms. Moreover, the current USP apparatus, initially designed for in vitro release of oral and transdermal products, are not directly applicable for extended release parenteral dosage forms. For example, USP apparatus 1 (basket) and 2 (paddle) based on the sample-and-separate methodology typically require large volumes of release media, which are not suitable for many low dose parenteral formulations. USP apparatus 3 (reciprocating cylinder) was designed for bead-type delivery systems. USP apparatus 4 (flow through cell) was designed for extended release oral dosage forms and as such is more applicable for extended release parenteral dosage forms compared to the other compendial apparatus (refer below). USP apparatus 5 (paddle over disc) and 6 (cylinder) that were designed for transdermal formulations are not desirable for in vitro release testing of polymeric microparticulate formulations, since the microparticles cannot be easily retained in these apparatus. USP apparatus 7 (reciprocating disc) was designed for transdermal systems and non-disintegrating extended release oral dosage forms. USP apparatus 7 has been used for some extended release parenteral dosage forms such as drug-eluting stents.

Over the past few years, extensive efforts have been made to develop suitable in vitro release testing methods for extended release parenteral dosage forms.^{[14; 16; 65; 42]} Since accelerated in vitro release testing can be used to rapidly assess and predict “real-time” drug release profiles, it is recommended that accelerated in vitro release testing should be developed as early as possible in the formulation development process.^{[19; 75; 68]} The current in vitro accelerated release testing methods used for extended release parenteral dosage forms include sample-and-separate methods, continuous flow cell methods as well as dialysis methods.

Sample-and-separate methods

This method is the most widely used research method for polymeric microparticles and implants.^{[14; 61; 8]} Conventionally, polymeric microparticles and implants are introduced into a vessel/vial containing release media and release is assessed over time. In the case of polymeric microparticles, centrifugation of the release media followed by sampling of the supernatant is widely used. In some cases, release media replacement may be necessary in order to maintain sink conditions or avoid drug degradation in release media.^[83] Different experimental setups (such as size of container, agitation, and sampling methods) can affect the in vitro drug release profile.

Accelerated methods based on the sample-and-separate methodology at acidic pH (pH 4) and elevated temperature (50°C) were developed to accelerate leuprolide acetate release from PLGA depot formulations.^[18] Complete release was achieved in 30-40 hours at 50°C compared to 42 days under “real-time” conditions. This method was able to differentiate different formulations and correlate well with “real-time” release at 37°C. USP apparatus 7 (reciprocating disk/stent holder) was utilized to develop an accelerated in vitro release method for biodegradable drug eluting stents. In this study, acetonitrile was added into the release medium to achieve over 80 % of Everolimus release within 24 h.^[73]

Although the sample-and-separate methodology provides a direct and reasonably accurate assessment of in vitro drug release, there are some limitations associated with this method such as inadequate agitation, loss of microparticulates during sampling, and the use of vials/vessels of different dimensions that makes intra-laboratory comparison difficult.^{[81; 14]}

Continuous flow cell methods

USP apparatus 4 (continuous flow cell method) was originally developed for extended release oral dosage forms. Modifications of the USP apparatus 4 have been used to assess drug release from extended release parenteral dosage forms such as microspheres,^{[81; 84; 85]} liposomes,^[86], drug-eluting stents and implants.^{[16; 69]} As shown in Figure 4, parenteral dosage forms have been placed into flow through cells together with glass beads and/or adaptors. For example, microspheres have been loaded into flow through cells with glass beads to prevent microsphere aggregation and facilitate laminar flow of the release media throughout the cells.^[81] A dialysis adaptor has been developed to hold nanoparticles such as liposomes within the flow through cells. The release media is circulated through the flow through cells and drug release is monitored from the effluent (open system) or the external media reservoir (closed system, Figure 4).

Figure 4.

USP apparatus 4 can simulate the in vivo environment such as subcutaneous tissue, since small volumes of media can be used and constant circulation can mimic the dynamic in vivo environment. In addition, the media volume used with the USP apparatus 4 can be modified to allow testing of various formulations, and this is particularly important for many low dose parenteral formulations. Elevated temperature and acidic pH accelerated release tests using USP apparatus 4 have been developed for rapid evaluation of microsphere formulations.^{[22; 15; 34]}

Dialysis methods

Dialysis methods appear to be an attractive option to study drug release from polymeric microparticles or in situ depot-forming systems. This method has been used to study drug release from oily parenteral depots,^{[87; 82]} microspheres,^{[12; 40]} liposomes,^[88] and implants.^[89] Among the dialysis methods, a rotating dialysis cell model and a Float-A-Lyzer® method have been used (Figure 5).^{[75; 90]} Accelerated drug release testing using the Float-A-Lyzer® was performed at high temperatures to investigate leuprolide acetate release from PLGA microspheres.^[75] A good correlation between the accelerated release profile and the “real-time” release data was obtained in this study.

Figure 5.

In order to develop suitable dialysis methods for in vitro release testing of the extended release parenteral dosage forms, the following parameters must be considered: agitation conditions; donor and acceptor cell volumes; and dialysis membrane molecular weight cutoff. It is recommended that the inside volume of the dialysis sac should be at least 6-10 fold less than that of the outer release media in order to provide a driving force for drug transport through the dialysis membrane.^[14] Furthermore, if the drug binds to the dialysis membrane, this technique is not applicable.

There are several disadvantages associated with the dialysis methods such as: i) violation of sink conditions within the dialysis sacs; ii) lack of agitation within the sacs that can lead to aggregation and consequent change in release profiles; and iii) since this is non-compendial method different laboratories may use different setups which can result in different release profiles.

Mathematical models

Beside the experimental release testing methods as outlined above, a mathematical model is also desirable to evaluate whether accelerated test data are predictive of “real-time” release profiles. The use of the Weibull function in modeling drug release from extended release dosage forms (e.g. biodegradable microspheres) was recommended at a previous AAPS/FIP workshop.^[11] The Weibull function assumes that the drug release is governed by polymer erosion coupled with minimal initial burst release as well as minimal diffusive release. This equation was used to model the drug release from PLGA microspheres (Equation 3) under accelerated and “real-time” testing conditions.^[75]

$$X∕X_{\mathrm{lnf}}=1-e^{-\alpha t^{\beta }}$$ (3)

where X is the percentage of drug released at time t and release is complete when X_inf is 100 %, α is a scale factor corresponding to the apparent rate constant, and β is a shape factor. The shape of the simulated curve can be characterized as exponential (β=1); sigmoid or S-shaped with an upward curvature followed by a turning point (β>1); or parabolic, with a higher initial slope and after that consistent with exponential (β<1).^{[91; 75; 29]}

Considering that an initial burst release phase may occur for polymeric microspheres, the Weibull equation has been modified to include the initial burst release phase:^[75]

$$X∕X_{\mathrm{lnf}}=X_{\text{burst}}∕S_{\mathrm{lnf}+(1-e^{-\alpha t^{\beta }})}$$ (4)

where X_burst is the percentage of drug released in the burst phase.

Analysis of the goodness of fit (R²) of model parameters α and β at accelerated and “real-time” conditions can identify accelerated parameters to optimize appropriate accelerated conditions to correlate with “real-time” release. These will facilitate the rational design of accelerated in vitro drug release testing method to serve as a reliable quality control tool.

Although the Weibull function is considered to be one of the most powerful mathematical models for the description of drug release kinetics in either Euclidean or fractal spaces,^[10] it is an empirical model that is not deduced based on kinetics and therefore cannot adequately characterize drug release kinetics.^[91]

Conclusions

Extended release parenteral dosages forms have been successfully used for the treatment of a variety of diseases. In order to assure the performance and safety of these products, the development of a suitable in vitro release testing method is crucial. Over the past decades, extensive efforts have been made to understand drug release mechanism(s) from these formulations. However, due to their differing complexities, the in vitro drug release mechanism(s) from such systems (e.g. lipid microparticulate systems) are far from fully elucidated. Consequently, it is very challenging to develop suitable in vitro release testing methods for these systems. Modification of standardized technologies or conventional methods and corresponding method validation may be necessary in order to accommodate the special characteristics of these formulations.

“Real-time” in vitro release testing is necessary to gain a mechanistic understanding of drug release and to develop a good in vitro-in vivo correlation, whereas accelerated release testing for extended release parenteral dosage forms is essential for quality control purposes as well as to assist in formulation development. Ideally, “real-time” and accelerated tests should follow the same release mechanism. However, for quality control purposes an accelerated test that follows a different release mechanism may be acceptable as long as it can discriminate out of specification batches and meets other required criteria. In the case of dosage forms that show an initial burst release, it may be necessary to conduct an initial “real-time” study to assess the burst release as this may be lost in the accelerated test. It is also important that the accelerated test should mimic the physiological conditions at the site of administration to the extent possible.

Other factors that should be taken into consideration during the method development process include stability of release media components and of the drug, as well as the robustness of the test apparatus to withstand the applied extreme conditions.